True, I was tending to ignore latency. Although it did cross my mind that if XDR's behaviour in this regard is a little better than GDDR3, it might help balance out a little.
You could flip it over and have the framebuffer in GDDR3, but I was putting it in XDR because of the greater amount of bandwidth there. With my same figures above, there'd still be some GDDR3 BW left over for other things, but it seems a little odd to me to have so little going into relatively so much memory. That also assumes then texture/vertex reads etc. are less bandwidth sensitive and could sustain any greater penalty going over to XDR..
But yeah, it is just playing with numbers for now. There are a number of factors that could render all this moot (hopefully in a good way
).